A fluorine gas is a combustion-supporting gas having strong oxidative property and also has strong toxicity and corrosiveness and therefore, in Japan, fluorine gas is scarcely traded on a commercial base except in a special case. Japanese International Application Domestic Publication No. 5-502981 describes a halogen generator, where fluorine (a halogen) is stored in the form of being occluded into a solid and on use of fluorine, the container housing the solid is heated to generate fluorine (a halogen). In this publication, it is stated that when K3NiF7 is used as a substance for occluding fluorine, fluorine having a purity of 99.7% can be continuously supplied. In general uses, a fluorine gas having a purity on the order of 99.7% can be used, however, in the field of production of semiconductors, for example, a fluorine gas having a high purity of 99.7% or more is demanded.
On the other hand, the fluorine gas being supplied on a commercial basis generally contains about 1.5 vol % of impurities. The majority of the impurities are N2, O2, CO2, fluorocarbons such as CF4, and gases such as SF6, SiF4 and HF. In the case of using a fluorine gas containing these impurities for the purpose of synthesizing a fluorine compound, these impurities have almost no effect and a purity of 98 to 99 vol % is sufficient. With respect to the method for analyzing a fluorine gas having a purity of 98 to 99 vol %, a volumetric titration method of allowing a fluorine gas to be absorbed in an aqueous KI solution and measuring liberated I2 using a Na2S2O3 solution, and an Orsat method of allowing a fluorine gas to react with and be absorbed in a KI solution and analyzing the purity from the volume of undissolved gas are known. In the Orsat method, when undissolved gas collected is analyzed by gas chromatography utilizing the low solubility of impurities such as N2, O2, CO2, fluorocarbon (e.g., CF4) and SF6 in a KI solution, the composition of the impurities can also be analyzed.
However, these analysis methods cannot be an optimal method as the analysis method for a high-purity fluorine gas, containing trace impurities in a concentration of hundreds of vol. ppm or less, even though such high-purity fluorine gas is an important key technology for the development of semiconductor industry. Fluorine gas is difficult to handle because it is highly reactive (oxidative) and in turn highly corrosive. Therefore, fluorine gas cannot be introduced directly into an analyzer. Even in the case of gas chromatography which is effective for analyzing the composition of gas components, almost no method has been heretofore known for the analysis of trace impurities in a fluorine gas, because, for example, there is no appropriate separation column where fluorine gas can be directly introduced.
Fluorine gas is used as an etching gas or a cleaning gas in the semiconductor industry because of its reaction properties. Particularly, in uses for annealing metal fluoride for optical materials or as a gas for an excimer laser, the optical properties of fluorine are also important and the amount of fluorine gas used for this purpose is increasing. Accompanying these demands, a high-purity fluorine gas and an analysis method therefor are strongly required. For optical uses, a high-purity fluorine gas reduced in impurities such as N2, O2, CO2, fluorocarbon (e.g., CF4), SF6, SiF4 and HF, and having a purity of 99.9 to 99.99 vol % is demanded. In particular, a high-purity fluorine gas having an O2 gas concentration of several vol ppm or less and a purity of 99.99 vol % or more is demanded.
Japanese Unexamined Patent Publication No. 4-9757 (JP-A-4-9757) describes a technique where a fluorine gas containing impurities is passed through a metal chloride-filled layer to convert the fluorine gas into a chlorine gas, the chlorine gas is fixed and removed by reacting the chlorine gas with an alkali metal aqueous solution and a metal or separated and removed by the adsorption to a porous polymer, and the separated impurities are analyzed by a gas chromatography. Also, Japanese Unexamined Patent Publication No. 7-287001 (JP-A-7-287001) describes a technique of heating cobalt difluoride (CoF2) at 200 to 300° C. to fix a fluorine gas as cobalt trifluoride (CoF3) and analyzing trace impurities separated from the fluorine gas using a gas chromatography.
In the reaction with a fluorine gas, both the metal chloride (NaCl) and cobalt difluoride (CoF2) show a low reaction rate at room temperature and to attain a complete reaction, a temperature of 100 to 300° C. is necessary. However, in these methods, for example, at the time of fixing a fluorine gas as a metal fluoride by the displacement with chlorine of a metal chloride, O2, which is one component of impurities in the fluorine gas, is generated. It is also found that if the sampling and sample measuring tubes, containers for filling metal chloride, flow path changeover valve and the like are not subjected to an inner surface treatment, generation of O2 and HF occurs, which is considered to be ascribable to the water adsorbed to the metal inner surface. This phenomenon cannot be overcome merely by a baking treatment and, due to the increase of O2 background, these methods cannot be accurate as a quantitative analysis method of trace oxygen gas in a fluorine gas.
The method described in JP-A-4-9757 is a method where the chlorine gas generated by the conversion of a fluorine gas is absorbed and reacted in an aqueous solution of alkali metal hydroxide and thereby removed and separated and thereafter, the impurities are analyzed by gas chromatography. In the case of analyzing trace impurities, the dissolution and absorption of impurities in the aqueous solution raise a problem and in some cases, the quantitative analysis cannot be exactly performed. If the purpose is to analyze the impurities in a chlorine gas, the gas chromatography method described in this patent publication may be a general and good method, where a chlorine gas is separated from impurities, using a separation column packed with porous polymer beads, by a pre-cut or backflash system and if desired, a separation column such as MS-5A is employed. However, the problem of impurities, particularly oxygen, generated in the previous stage from the inner surface of equipment or material coming into contact with the fluorine gas or from the fluorine gas-removing and separating agent still remains unsolved.
If this problem can be solved, H2, O2, N2, CH4, CO, CO2, fluorocarbon (e.g., CF4) and SF6 out of impurities in a fluorine gas may be analyzed by the analysis method described in those patent publications. However, other than these impurities, the fluorine gas contains impurities such as HF, SiF4 and other metal fluorides and unless these impurities can be analyzed, the method cannot be said to attain a function as an analysis method of trace impurities.